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Threats to tear up Trans-Pacific Partnership have damaging implications for our trade.

Donald Trump, winner of the Republican race in New Hampshire by a country mile, tells voters he can make America "great again" with some simple fixes.

He will deal with unemployment by being "the greatest jobs president". His Middle East strategy rests on knocking "the hell out of Isis". Drug abuse in the United States is "going to be over. We'll get it done", the real estate tycoon assures his crowded rallies. All this and more will be achieved with the clever people around him. They'll get it done, he insists, "believe me".

Trump's resounding win in the Granite State suggests more and more Americans are prepared to believe him rather than the candidates who Republican party managers had anticipated would be leading the pack at this point in the US presidential campaign.

On the Democratic side of the fence, the fortunes of the once-favoured Hillary Clinton have dived. Her humiliation at the hands of Bernie Sanders is not the end of the road for her but she faces a tough battle to halt her 74-year-old opponent's momentum and his pitch-perfect targeting of Wall St excess and the clout of big banks.

Clinton, too, says she'll curb the finance sector but her history of receiving support from big institutions has left her wounded.

Conventional wisdom suggests her sagging prospects will recover as the campaign shifts to the west and south where rich lodes of Latino and African-American votes reside.

But the one certainty in this contest is that Sanders and Trump, the runaway leaders, have tapped into a well of anger and unrest in the US electorate that has confounded predictions and derailed seemingly more electable contenders.

Democrats in New Hampshire embraced a socialist.

The one certainty ... is that Sanders and Trump, the runaway leaders, have tapped into a well of anger and unrest in the US electorate that has confounded predictions.

The state's Republicans picked a demagogue. Americans appear to be saying they want change, but are heading in different directions in search of it. Democrat voters supporting Sanders come from the left and from the ranks of younger Americans who feel they are falling behind economically. Trump, too, is finding his message resonates with voters who complain they are being pushed aside, especially among less-educated white communities.

Daylight separates the front-runners on social policy, where Trump proposes barring non-citizen Muslims entering the US, and removing as many as 11 million undocumented immigrants.

But the rivals share common ground in a key area of economic and trade policy which, if it comes to pass, could damage New Zealand's interests.

Both men want to tear up free trade deals, including the Trans-Pacific Partnership which the US, along with 11 other nations around the region, signed in Auckland last week. New Zealand's prosperity rests on the ability to get exports into markets with as few impediments as possible.

Sanders and Trump alike complain that trade deals signed by Washington over the years have come at the expense of American jobs. This has been an argument against trade liberalisation all along, but the political consensus that everyone benefits from free trade has prevailed.

Now this policy is under fierce attack, with Trump and Sanders accusing the US political establishment of opening up American markets without extracting equal concessions from trade partners.

Trump also proposes a steep tariff on Chinese imports - a move which would invite Beijing's retaliation. The implications for the global economy - and for New Zealand - could be profound. The White House race clearly bears watching.